[MTD] fix CONFIG_MTD_SHARP_SL if CONFIG_MTD=m

Sharp Zaurus SL-C3200 with CONFIG_MTD=m and CONFIG_MTD_SHARP_SL=y (as it
is bool) lost support for the ROM flash. With CONFIG_MTD=y it has no
problems.

It is caused by losing of compiled code of
drivers/mtd/maps/sharpsl-flash.o.
It was linked to drivers/mtd/maps/built-in.o and drivers/mtd/built-in.o,
but lost and not linked to drivers/built-in.o (because CONFIG_MTD!=y).

Patch below fixes this problem by creating sharpsl-flash.ko (and the
code works correctly as a module).

Signed-off-by: Stanislav Brabec <utx@penguin.cz>
Signed-off-by: David Woodhouse <dwmw2@infradead.org>